This invention relates to glass to metal seals for use in the hermetic encapsulation of active and passive components in electrical engineering and electronics. These units are typically made out of metal (baseplate) having one or several bores for accommodating sealing glass performs and lead-through conductors of a metal alloy more or less dilathermally matched to the sealing.
All members are fused hermetically to form a matched or compression glass to metal seal.
In FIG. 1 there is schematically illustrated a conventional glass to metal seal of the type discussed above, and wherein 1 denotes the baseplate, 2 denotes the conductor or wire lead, and 3 denotes the insulating glass.
In the prior art, in order to fuse the conductor into the bore of the baseplate, all parts are supported on a graphite plate and then subjected to a heat treatment in an appropriate furnace in a protective or inert gas environment. During this process, the glass is softened, fills the bores fully, and bonds with the metals of the baseplate and the lead-in wire to form a hermetic seal.
However, in this method of manufacture and construction it is also generally unavoidable to have the glass bead employed, as a result of sagging, come into contact with the supporting plate of graphite. This is caused by gravity and as a result, loose carbon particles become incorporated into the glass surface or become attached thereto during the fusing or melting step. Hereinafter the terms "fusing" and "melting" will be used interchangeably and will refer to the melting and bonding of the glass insulator within the baseplate bores. Incorporated or adherent graphite particles can cause problems in certain specific applications, for example, when the glass to metal seals are nickel-plated, because electro-deposited metallic particles can attach themselves to the graphite particle thereby impairing the electrical properties of the glass to metal seal (flash over voltage, insulating resistance).
Attempts have been made to avoid this sagging through of the normally cylindrical sintered glass bead by providing the bead with a collar to fix or hold the bead in position during the fusion process until it has bonded to the metal wall. The adherence of the glass bead at the edge of the eyelet bores hinders the softened glass to sag totally in the bore and thus a direct contact of the glass with the graphite surface is avoided. Such a modification is shown in FIG. 2, wherein 4 denotes the graphite plate and 5 denotes the glass bead with a collar.
This manufacturing method, however, also has disadvantages. Two serious disadvantages in this method are:
1. A collared bead is more difficult to produce than a cylindrical glass bead; and
2. The use of the collared bead requires a special alignment procedure during assembly which creates difficulties especially in automated assembly installations.